This invention relates to artificial ligaments. In a particular aspect, the invention relates to woven artificial ligaments for replacement of the cruciate ligaments of the knee.
The use of synthetic fibers for the construction of prosthetic ligaments and tendons is well-known. These fibers are usually woven or braided into a cylindrical tube or cord structure, which is designed to mimic the flexibility, load-percent elongation and strength characteristics of the natural ligament. The woven structure also provides a porous matrix for tissue and bone ingrowth and anchorage to the bone.
Current surgical procedures for implantation of prosthetic ligaments usually involve boring tunnels through the articular ends of the bones, passing the prosthetic ligament through these tunnels, and anchoring them to the outside of the bone using staples, screws or the like. See FIG. 1. Such prosthetic ligaments generally must meet a wide range of performance requirements, including biocompatibility, high fatigue life, and mechanical properties appropriate to stabilize the involved joint. Currently available prosthetic ligaments do not possess mechanical properties similar to natural ligaments.
Artificial ligaments implanted by the above-described procedure can be divided into several distinct regions. Each of these regions poses specific design problems and requirements. The intra-articular region is that segment of the ligament which lies between the articular ends of the bones. This region advantageously is of minimal cross-sectional area and exhibits flexural properties and stress-strain behavior similar to that of the natural ligament being replaced. In addition, this region preferably has an ultimate tensile strength higher than that of the natural ligament. The intra-articular region includes the portions of the ligament which contact the entrances to the bone tunnels. Thus, this region should also exhibit a high degree of abrasion resistance.
The tunnel region of the artificial ligament is in direct contact with the bored tunnels in the bone. Important considerations for this region include the ability to be closely conformed to the shape of the bone tunnel, a minimal elongation under load, and an appropriate degree of porosity. All of these features enhance the propensity for bone ingrowth and hence permanent and strong fixation of the device to the bone.
The end regions of the artificial ligament provide a site for mechanical attachment of the ligament to the bones. A secure mechanical attachment is important to provide a stable configuration until permanent fixation through tissue and bone ingrowth has occurred. These regions should be quite stiff (minimal elongation under load) and preferably should include features which expedite temporary fixation by staples, screws or other fasteners.
Various designs have been proposed for meeting the rigorous requirements of a prosthetic ligament. For example, Rothermel et al., U.S. Pat. No. 4,255,820, describe an artificial ligament woven from polyester textile fibers. This artificial ligament, which is tubular in shape, is divided into three zones: zone A is the center segment of the ligament which lies between the articular bone ends; zone B is a transitional region where the ligament enters the bone tunnels; and zone C is the region which resides in the bone tunnels. The Rothermel ligament does not have a segment for mechanical attachment to the bone, but instead, relies entirely on anchorage by tissue and bone ingrowth.
The various zones of the Rothermel ligament are characterized by different diameters and porosities. The differences in porosity are achieved by varying the weave structure along the longitudinal axis of the ligament. The center section (zone A) has a very tightly woven structure to prevent tissue ingrowth. This section is said to be "elastic, having an elongation factor of between 4% and 6% of its length." Rothermel et al. do not define the terms, "elastic" and "elongation factor." The center section may also contain a rubber or Dacron core to provide stress relief. The transitional zone has an intermediate pore size to permit soft tissue ingrowth, but not bone ingrowth. The end zones are flared to allow insertion of an autogenous bone graft and to resist extraction into the joint space when under stress. The end zones are relatively loosely woven to create an increased porosity conducive to bone ingrowth.
Varying weave densities and pore sizes, using the same types of fibers throughout the length of the synthetic ligament, as described by Rothermel et al., is said to enable the zones to function differently in regard to wear, fraying, fatigue, and anchoring. However, the extent of differences in physical characteristics which can be achieved using this technique is limited. A need exists for a prosthetic ligament which has different zones whose physical properties are tailored to the particular function they are to serve in the body.